1. Field of the Invention
The present invention relates to a cold cathode, and, more particularly, to a field emission device such as a using a vacuum microelectronics technology.
2. Description of the Related Art
FIG. 1 presents a cross section exemplifying a conventional field emission device, called the Spindt type, belonging to field emission device that use a vacuum microelectronics technology. As shown in FIG. 1, formed on a semiconductor substrate 1 are a cone-shaped emitter 2 that will serve as a minute emitters, an insulating layer 3, and a gate 4 formed of a metal layer. A positive voltage--positive with respect to the potential of the emitter that is the same as that of the semiconductor substrate 1--is applied to this gate 4. As the tip of the emitter 2 is very sharp, it is applied with a high electric field and thus emits electrons which correspond in an amount to the voltage applied to the gate 4.
The amount of current to be emitted from a single emitter 2 is as much as 10 to 100 .mu.A. For use as the cathode of an electron tube, a number of emitters 2 are arranged two-dimensionally (see, for example, Journal of Applied Physics, Vol. 47, 1976, No. 12, pp. 5248 to 5263).
In an electron tube such as a traveling-wave tube or a cathode ray tube, when gas remaining in the electron tube hits an electron beam, it is ionized and the positive ions are accelerated through the center of the electron beam in the opposite direction to that of this beam, finally hitting the center portion of the cathode. This bombardment of the positive ions at the center portion of the cathode may destroy the emitters 2 in this area, stopping electron emission therefrom. The reduction in electron emission from the center portion decreases the cathode current accordingly, deforming the shape of the electron beam. Such phenomenon definitely makes the function of the electron tube unstable as the time passes. Further, the ion bombardment may cause melting or sputtering of the material of the cathode and mechanical deformation of the emitter 2 and gate 4, resulting in the electric short circuit of the emitter 2 with the gate 4. This disables the application of a voltage between the emitter 2 and gate 4, preventing electron emission from the whole cathode.